1. Field of the invention.
The present invention pertains generally to optimization of photoimage and plating processes for printed circuit board manufacturing.
2. Description of the Background.
Electrochemical deposition processes are used in many different manufacturing processes. An electrochemical process typically comprises a plating bath, an anode, and a cathode. The plating bath is a solution of a number of chemicals. Not only does each of the chemicals affect the electrochemical properties of the process, so do the various combinations of the chemicals and their interactions. Also, many other factors individually and in combination affect the process. Some of the other factors are anode placement, cathode placement, temperature gradients, current density gradients, the fluid flow of the solution and ion mass transfer at the surface, composition of the materials used in the plating bath container, and various contaminants. Plating baths, even of the same solution, are individual and distinct. Because of the complexity of the variables affecting the electrodeposit, the electroplating process is very difficult to control only by chemical analysis of the electroplating baths alone.
In order to achieve uniform and predictable electrical and mechanical properties of the electrodeposit, the electroplating processes need to be characterized and controlled. One way of doing this is to use a Hull cell. A Hull cell is a small volume of plating solution to be characterized with a test panel containing a polished metal surface as a cathode. The polished metal surface is positioned at an angle to the anode, in order to cause plating at different current densities along the panel. The current densities for different distances from the vertical edge of the panel were calculated indirectly, by the weight of the deposit, and assumed to be constant for given current. By determining the appearance, and generally the obscurity, of the electrodeposit, which depends on the current density and the bath composition, the bath can be characterized.
One limitation of the Hull cell was its failure to measure directly the current distribution on the cathode. The assumption that the current densities for different distances from the edge of the panel are constant for given current is not entirely correct. The current distribution is not only a function of the current applied, but also a function of the bath composition. An improvement was made to the Hull cell in which the cathode was segmented so the current through each segment could be monitored. One way of monitoring the segmented cathode is to put a resistor between each segment and the common power supply. The voltage drop across the resistor is directly related to the current going through that segment by Ohm's Law. A data acquisition instrument like the HP 3497 can be used to scan and monitor the voltage drop which indicates the current density corresponding to each segment during the plating process. In this way, the effects of current density distribution on the electrodeposit and the plating efficiency can be determined. Also, a current versus time graph can be plotted. In this way, the saturation condition can be determined where the plating process becomes diffusion-limited for that segment of the cathode.
Although the segmented Hull cell is useful, in printed circuit board manufacturing it has many limitations. The Hull cell, whether segmented or not, fails to determine the effects of factors introduced by the cathode when the cathode has a pattern of photoresist for plating and nonplating areas, and when the cathode has different width plating areas for each segment. The segmented cathode fails to show the effects of the solution on the photoresist and the effect of photoresist on the electrodeposition in plating areas separated by photoresist.
A segmented cathode with a checkerboard pattern and a gross circular plating feature was developed. These relatively large patterns were used to determine the stability of the resist by the number of photoresist squares which could be pulled off using a standard adhesive tape. The plating uniformity could be established as in the above designs by measuring the thickness of the metal in the gross circular features and by visual inspection of the quality of the plating. This design has limitations since it does not show the effect of photoresist on small geometries and the effect of microcontamination in immediately adjacent plating areas. It also does not show the effect of the photoresist post-development cleaning on the metal deposition and absorption of the photoresist cleaner on lateral surfaces of photoresist subsequently contaminating the metal. Another limitation is inability to determine the effects when the photoresist on the cathode introduces contaminants and fluid flow perturbations within small areas of the surface of the cathode.
Printed circuit boards are complicated cathodes, involving complex designs of interconnect conductor networks for boards that may eventually contain and connect thousands of components or integrated circuit packages. Also, the cathode may be subjected to a myriad amount of processes during the photoimaging. The photoimage process involves applying a pretreatment to the surface for adhesion of the photoresist, exposure of the photoresist, development of the photoresist, rinse or development stop, baking of the photoresist, post-development clean of the photoresist, and other processes peculiar to particular photoimaging techniques. The cathode surface is directly or indirectly exposed to these processes and the effects on the cathode surface may affect subsequent plating processes. These effects need to be controlled and tested for in order to characterize the plating process. All the present Hull cell designs fail to provide complete and accurate characterization of the current density distribution on the surface of the cathode as a function of cathode line width, of the effect of the fluid agitation on the complex patterns of the cathode, of the effect of the previous cleaning of the printed circuit board and of the plating on the stability of the photoresist pattern on the cathode masking areas from the plating process, and of the effect of the previous cleaning of the printed circuit board and the photoresist pattern on the plating process. Further, no present design provides a quantitative measure of the resolution of the combined photoimaging and plating processes.